Google OAuth2 in Clojure

Recently in my Clojure project we had to integrate with Google's OAuth for
authenticating our users. It was a pretty painful process, so here's a quick
guide for anyone that wants to do this in the future.

For this guide I will be using stuarth/clj-oauth2 "0.3.2". First add that as
a dependency to your project.clj file. Next lets create a authentication
module.

(ns authentication
  (:require
    [cheshire.core :refer [parse-string]
    [clj-oauth2.client :as oauth2]))

(def login-uri
  "https://accounts.google.com")

(def google-com-oauth2
  {:authorization-uri (str login-uri "/o/oauth2/auth")
   :access-token-uri (str login-uri "/o/oauth2/token")
   :redirect-uri "http://localhost:8080/authentication/callback"
   :client-id "CLIENT"
   :client-secret "CLIENT-SECRET"
   :access-query-param :access_token
   :scope ["https://www.googleapis.com/auth/userinfo.email"]
   :grant-type "authorization_code"
   :access-type "online"
   :approval_prompt ""})

(def auth-req
  (oauth2/make-auth-request google-com-oauth2))

(defn- google-access-token [request]
  (oauth2/get-access-token google-com-oauth2 (:params request) auth-req))

(defn- google-user-email [access-token]
  (let [response (oauth2/get "https://www.googleapis.com/oauth2/v1/userinfo" {:oauth access-token})]
    (get (parse-string (:body response)) "email")))

;; Redirect them to (:uri auth-req)

;; When they comeback to /authentication/callback
(google-user-email  ;=> user's email trying to lgo in
  (google-access-token *request*))

So what did we do here? First of all we required the OAuth2 dependency into our namespace. We also included cheshire, Clojure's JSON parsing library. Then we created a hash google-com-oauth2. This hash contains all of the information Googleneeds when we request a OAuth2 access token. Replace the :client-id and :client-secret with the values you get from Google when you set up your Google application. Also be sure that your :redirect-uri matches the one you supplied Google.

Using this data has we can construct a auth-req using our OAuth2 library. When users go to our application, when they try and log on the app should redirect them to (:uri authentication/auth-req).

When the user gets back to our application it will be at out callback uri.
The request params of this request should look like,

{:code "4/dasfjkhadsfkalsdasdfaskjf}

Using this request object we can get back a access-token from Google. Finally once we have an access token, we get start making oauth/get's to retrieve user info from Google. I've written the method google-user-email, but you can get other values from the user if you change the scope of your request.